The present invention is related in general to the field of electronic circuits, and more specifically to an apparatus and method for reducing silicon area of a loop filter included in a phase locked loop (PLL) without increasing noise.
A PLL is a well-known electronic circuit used in many semiconductor devices. A PLL is a closed loop feedback control circuit which provides an output signal that is locked in phase and frequency of an input signal used as a reference. FIG. 1A is a simplified block diagram of a type II phase locked loop 100, according to prior art. The PLL 100 includes a phase frequency detector (PFD) 110, a charge pump (CP) 120, a loop filter 130, a voltage-controlled oscillator (VCO) 140, and an optional divider 150. The PFD 110 compares a feedback signal 112 received from the divider 150 with a reference signal 102 and generates an error signal 104 which is proportional to the magnitude of the phase/frequency difference between them. The error signal 104 is provided to the CP 120. The CP 120 provides a current output to control a magnitude of the charge stored in the loop filter 130, thus converting the output of the PFD 110 to a control voltage input 106 recognizable by the VCO 140. The VCO 140 generates an output frequency signal 108 proportional to the control voltage input 106. The output frequency signal 108 may be optionally further divided down by the divider 150 before being fed back to the PFD 110. When the PLL 100 is “locked”, there is a constant phase difference (usually zero) between the feedback signal 112 and a reference signal 102 and their frequencies are matched.
It is well known that the loop filter 130 may be implemented using passive components such as a passive resistor capacitor (RC) circuit or may be implemented using an active component such as an operational amplifier (OA or opamp) used in combination with an RC circuit. A large value of a capacitor may be required to provide a lower zero frequency of the loop filter 130. In addition, the large value of the capacitor may also be used to help reduce the value of the resistor and hence the phase noise. However, a capacitor having a large value consumes a significant portion of silicon chip area. In some conventional filters, the capacitor having the large value may be fabricated off-chip, e.g., as an externally mounted device.
FIG. 1B illustrates a traditional loop filter 160 having an input 162 and an output 164, according to prior art. The loop filter 130 described with reference to FIG. 1A may be implemented as the traditional loop filter 160. The traditional loop filter 160 is implemented with an OA 166 and a RC circuit. The RC circuit includes a resistor R 152 coupled in series with a capacitor 154, the resistor R 152 being coupled to the input 162 and the capacitor 154 being coupled to the output 164. A capacitor C 156 is connected between the output 164 and the input 162 to filter high frequencies. Active filters such as the traditional loop filter 160 may be used in processes having high device leakage or in cases where voltage at the CP 120 output needs to be substantially constant.
FIG. 1C illustrates a prior art loop filter 170 having multiple active components, according to prior art. The loop filter 130 described with reference to FIG. 1A may be implemented as the prior art loop filter 170. The loop filter 170 is substantially similar to the traditional loop filter 160 except for an additional transconductance amplifier 172. The loop filter 170 includes the resistor 152 coupled in series with the capacitor 154, the resistor R 152 being coupled to the output 164 and the capacitor 154 being coupled to the input 162. Inputs of the transconductance amplifier 172 are coupled across the resistor R 152 and the output of the transconductance amplifier 172 is coupled to the input 162. The loop filter 170 increases a value of the resistor R 152, thereby reducing a value of the capacitor C 154, by adjusting the transconductance amplifier 172. However, traditional and prior art loop filters such as the loop filters 160 and 170 which use active components are susceptible to phase noise generated by the resistor R 152, especially phase noise generated within a bandwidth of the filter. The phase noise is typically dependent on a selected value of the resistor R 152, which affects the value of the capacitor C 154. Thus, traditional and prior art loop filters are often not able to control introduction of phase noise, while attempting to reduce the value, and hence the size, of the capacitor C 154.